Marine seismic data acquisition is generally conducted by a vessel which tows a seismic source, for example an airgun array, that periodically emits acoustic waves into the water in order to penetrate the seabed. Furthermore, ocean bottom cables being deployed on the seabed and comprising sensor units provided with hydrophones, geophones and accelerometers, detect the acoustic waves reflected off boundaries between layers in geologic formations. More particularly, hydrophones detect acoustic pressure variations while geophones and accelerometers detect particle motion caused by the reflected acoustic waves.
Marine seismic data acquisition may be permanent or non permanent.
In the permanent marine seismic data acquisition, the ocean bottom cables are left on the seabed for a long period of time, for example 25 years. During that time, the ocean bottom cables are connected to a recording unit which may be placed on the vessel, on a buoy or on an offshore platform.
In a non-permanent marine seismic data acquisition, the ocean bottom cables are left on the seabed for a limited period of time. The ocean bottom cables are similarly connected to a recording unit which may be placed on the vessel, on a buoy or on an offshore platform.
Different systems and methods exist for the deployment of an ocean bottom cable on the seabed for performing marine seismic data acquisition. These systems may be used for performing permanent and/or non permanent marine seismic data acquisition.
In a well-known system, a plurality of ocean bottom cables are deployed directly from the surface of a vessel to the seabed with the use of a vessel controlled winch or a linear cable engine. In this system, in order to record the data being detected by the sensor units of the ocean bottom cables, the ocean bottom cables have to be connected to a recording unit placed on the vessel during the performance of either a permanent or a non permanent seismic data acquisition. However, in extreme weather conditions (e.g. strong current and waves) an entanglement between the different ocean bottom cables mounted from the surface of the vessel to the seabed may be provoked and thus the cables might be damaged.
In another example of a cable deployment system and method being disclosed in the United States publication US2005/0276665A1, the cable deployment system uses a cage for transferring the ocean bottom cable from a vessel to the seabed. In an embodiment of this system, the cage is composed of an upper frame and a lower frame and is lowered by the vessel to the seabed by means of a vessel controlled winch and an umbilical attached to the cage. The lower frame of the cage is adapted to receive a reel which is rotatably and removably mounted within the same and the upper frame of the cage is adapted to be removably attached to the lower frame. Furthermore, the reel is adapted to receive an unspoolable length of ocean bottom cable. Once the cage is lowered proximally to the seabed, the vessel starts to move and thus the deployment of the ocean bottom cable on the seabed is achieved. Once the ocean bottom cable is deployed and the reel is empty, the cage may be retrieved by the vessel. Then, the upper frame may be disconnected from the lower frame and the empty reel may be recovered from the cage and be replaced by another reel comprising ocean bottom cable.
However, the above mentioned publication does not solve the problem of cable entanglement described above. Particularly, in the cable deployment system of the above mentioned publication, as well as in similar cable deployment systems wherein the ocean bottom cable is not directly connected to a recording unit placed on a vessel, a riser cable has to be used in order to connect the ocean bottom cable and the recording unit placed on the vessel during the performance of either a permanent or a non permanent data acquisition. Specifically, the riser cable is used to transfer the data being detected by the sensor units of the ocean bottom cable to the recording unit placed on the vessel. Accordingly, in the case of using a plurality of riser cables being connected to a plurality of ocean bottom cables, an entanglement between the riser cables may be provoked in extreme weather conditions and thus the riser cables might be damaged.